The present invention relates to a bipolar transistor and a method of fabricating the same, and more particularly, it relates to a bipolar transistor suitably used in mobile communication equipment and a method of fabricating the same.
Recently, a field effect transistor formed from GaAs with small power consumption (MESFET) is widely used as a transistor of a transmitting power amplifier used in mobile communication equipment such as a portable telephone. A negative power source is generally used for bias for a gate electrode of a MESFET. Accordingly, in using a MESFET in a transmitting power amplifier, two power sources, namely, a positive power source and a negative power source, are required. This is a disadvantage to downsizing of the amplifier, and hence, a transistor operated by using a positive power source alone is earnestly desired.
Furthermore, in recent communication systems such as CDMA (code division multi-channel access), an output current of a transmitting power amplifier is required to have small distortion (namely, to be linear). As a transistor meeting these requirements, a heterojunction bipolar transistor (HBT) including the emitter formed from a semiconductor having a larger band gap than a semiconductor forming the base is practically used.
As the materials for an HBT, GaAs and AlGaAs have been generally used for a base layer and an emitter layer, respectively, but InGaP having less surface recombination and higher reliability than AlGaAs is recently used for an emitter layer.
A performance index corresponding to a high operation speed of a bipolar transistor is a maximum oscillation frequency fmax, which is represented as follows:
fmax=(ft/(8xcfx80RbCbc))1/2 
wherein ft indicates a cut-off frequency, Rb indicates a base resistance and Cbc indicates a base-collector capacity.
In an HBT, even when the base concentration is high, sufficient current amplification can be obtained owing to the effect of band discontinuity of the valence band (xcex94Ev). Therefore, the cut-off frequency ft can be increased by reducing the thickness of the base as well as the base resistance Rb can be reduced by increasing the base concentration, so that the maximum oscillation frequency fmax can be high.
When an HBT is used at a frequency of approximately 0.8 through 2 GHz used for portable telephones and the like, however, maximum stable gain (MSG) at the frequency is more significant than the maximum oscillation frequency fmax.
FIGS. 10 and 11 show the results of simulation of the MSG at a frequency of 2 GHz of a conventional HBT. FIG. 10 is a diagram of dependency on the base-collector capacity Cbc of the MSG at 2 GHz of the conventional HBT, and FIG. 11 is a diagram of dependency on the base resistance Rb of the MSG at 2 GHz of the conventional HBT. In these graphs, the abscissas are standardized by an initial value CbcO of the base-collector capacity Cbc and an initial value RbO of the base resistance Rb, respectively. It is understood from the results that the MSG minimally depends upon the base resistance Rb but largely depends upon the base-collector capacity Cbc at a frequency of 2 GHz. Accordingly, in order to fabricate an HBT with large MSG, it is effective to employ a structure with a small base-collector capacity Cbc for the HBT. For attaining a small base-collector capacity Cbc in the HBT, it is effective to reduce the area of a region in the base where minority carriers are not injected from the emitter. Therefore, it is effective to dispose a base electrode between emitter regions in the sectional structure of the HBT. Now, a method of fabricating an HBT having such a structure will be described with reference to FIGS. 12A through 12D, 13A and 13B.
First, in a procedure shown in FIG. 12A, a collector contact layer 32 of n+xe2x80x94GaAs, a collector layer 33 of nxe2x88x92xe2x80x94GaAs, a base layer 34 of p+xe2x80x94GaAs, an emitter layer 35 of nxe2x80x94InGaP and an emitter contact layer 36 of nxe2x80x94GaAs and n+xe2x80x94InGaAs are successively deposited by epitaxial growth on a GaAs substrate 31. Then, a WSi film 37, that is, a metal film with a high melting point, is deposited thereon by sputtering.
Next, in a procedure shown in FIG. 12B, a resist (not shown) is formed on the substrate and is subsequently patterned. Then, an opening for exposing a surface of the emitter contact layer 36 is formed in the WSi layer 37 through reactive dry etching using the resist as a mask. Thus, the WSi layer 37 is formed into an emitter electrode 38 having the opening for exposing the surface of the emitter contact layer 36.
Then, in a procedure shown in FIG. 12C, the emitter contact layer 36 of nxe2x80x94GaAs and n+xe2x80x94InGaAs is patterned through etching using the emitter electrode 38 as a mask and a mixture of sulfuric acid, hydrogen peroxide and water as an etchant. At this point, the emitter layer 35 of nxe2x80x94InGaP is never etched by the etchant (the mixture of sulfuric acid, hydrogen peroxide and water). Specifically, the emitter contact layer 36 is patterned by completely selective etching in this procedure.
Next, in a procedure shown in FIG. 12D, a resist pattern (not shown) for defining a base region on the substrate is formed. By using the resist pattern as a mask, the emitter layer 35 is patterned through etching using an etchant of a mixture of hydrochloric acid and water. Thereafter, through etching using an etchant of a mixture of sulfuric acid, hydrogen peroxide and water, the base layer 34 is patterned and the collector layer 33 is partly etched.
Subsequently, in a procedure shown in FIG. 13A, a resist pattern (not shown) for forming a collector electrode on the substrate is formed. By using the resist pattern as a mask, an opening for exposing a surface of the collector contact layer 32 is formed in the collector layer 33 through etching using an etchant of a mixture of sulfuric acid, hydrogen peroxide and water. Then, a collector electrode 39 of AuGe/Au is formed by lift-off on the surface of the collector contact layer 32 exposed in the opening. Thereafter, a heat treatment is carried out at 450xc2x0 C., so that the collector electrode 39 can attain a good ohmic characteristic.
Next, in a procedure shown in FIG. 13B, a resist pattern (not shown) for forming a base electrode on the substrate is formed. By using the resist pattern as a mask, an opening for exposing a surface of the base layer 34 is formed in the emitter layer 35 through etching using an etchant of a mixture of hydrochloric acid and water. Then, a base electrode 40 of Ti/Pt/Au is formed by the lift-off on the surface of the base layer 34 exposed in the opening.
Through the aforementioned procedures, an HBT having the structure with a small base-collector capacity Cbc is completed.
Furthermore, in order to reduce surface recombination, which leads to decrease of the current amplification, on the interface between the emitter and the base of the HBT, the emitter layer 35 is generally formed from an emitter region 41 disposed below the emitter contact layer 36 and depleted emitter protection layers 42 and 43 formed in the periphery of the emitter region 41 as is shown in FIG. 14. The depleted emitter protection layers 42 and 43 are also designated as guard rings or ledges.
The conventional HBT described above has, however, a problem that sufficient MSG cannot be attained at a frequency of several GHz.
The present invention was devised to overcome the aforementioned problem, and an object is providing a bipolar transistor attaining large MSG and a method of fabricating the same.
The bipolar transistor of this invention comprises a collector layer; a base layer deposited on the collector layer; and a semiconductor layer deposited on the base layer in the shape of a ring along an outer circumference of the base layer, wherein the semiconductor layer includes a ring-shaped emitter region functioning as an emitter, and an outer edge of the emitter region and an outer edge of the base layer are disposed in substantially the same plane position.
Accordingly, the base area is reduced as compared with that of a conventional bipolar transistor. As a result, the bipolar transistor can attain a small base-collector capacity Cbc and large MSG.
The emitter region may be formed in the shape of a closed ring.
Alternatively, the emitter region may be formed in the shape of an opened ring.
The bipolar transistor can further comprise an emitter contact layer deposited on the semiconductor layer in the shape of a ring along the outer circumference of the base layer.
The semiconductor layer can include, on the inside of the emitter region, a ring-shaped inside protection layer projecting inward beyond the emitter contact layer.
The semiconductor layer can include, on the outside of the emitter region, a ring-shaped outside protection layer projecting outward beyond the base layer.
The semiconductor layer may have a larger forbidden band width than the base layer.
The semiconductor layer can be formed from InGaP and the base layer can be formed from GaAs.
The method of fabricating a bipolar transistor of this invention comprises the steps of (a) preparing a substrate having a first semiconductor layer, a second semiconductor layer deposited on the first semiconductor layer and a third semiconductor layer deposited on the second semiconductor layer; (b) patterning the third semiconductor layer and the second semiconductor layer by using a first etching mask formed over the third semiconductor layer; and (c) patterning the third semiconductor layer into a ring shape by using a second etching mask formed over the third semiconductor layer, wherein the first semiconductor layer, the second semiconductor layer and the third semiconductor layer are respectively formed into a collector, a base and an emitter.
Accordingly, the third semiconductor layer for forming the emitter and the second semiconductor layer for forming the base are patterned by using the same etching mask. Therefore, the outer edge of the emitter and the outer edge of the base are disposed in substantially the same plane position. Accordingly, the area of a region in the base where minority carriers are not injected from the emitter can be reduced as compared with that of a conventional bipolar transistor. As a result, the bipolar transistor can attain a small base-collector capacity Cbc and large MSG.
The substrate may further have a fourth semiconductor layer deposited on the third semiconductor layer in the step (a), the first etching mask is formed over the fourth semiconductor layer, and the fourth semiconductor layer, the third semiconductor layer and the second semiconductor layer can be patterned by using the first etching mask in the step (b), the second etching mask is formed over the fourth semiconductor layer, and the fourth semiconductor layer can be patterned into a ring shape by using the second etching mask in the step (c), and the method can further include a step (d) of patterning the third semiconductor layer into a ring shape by using a ring-shaped third etching mask formed over the third semiconductor layer and projecting inward beyond the fourth semiconductor layer in the shape of a ring.
A semiconductor used for forming the third semiconductor layer may have a larger forbidden band width than a semiconductor used for forming the second semiconductor layer.
The third semiconductor layer can be formed from InGaP and the second semiconductor layer can be formed from GaAs.